Thioredoxin H is a cytosolic member of the thioredoxin family of proteins. These small proteins (typical mass of 12 kD) have been shown to play a central role in the activation of proteins by influencing the redox status of sulfhydryl groups on target proteins (Holmgren et al. (1985) Annu. Rev. Biochem 54:237–271; and Holmgren et al. (1995) Structure 3(3):239–243). Two other thioredoxin classes, F and M, are located in plastids and have been shown to be involved in redox mediated activation/inactivation of various photosynthetic enzymes during light/dark transitions. The cytosolic (H) form of thioredoxin has been shown to be involved in disassembly of seed storage proteins during germination and in the bread making process. In the former case storage proteins are held together in clusters by S—S bonds. On germination thioredoxin H reduces the S—S bonds and the subunits dissociate, facilitating attack by proteases. During bread making the same processes occur. Reduction of the S—S bonds causes the protein complexes to disassemble allowing them to be distributed through out the dough during mixing. During kneading the S—H bonds become oxidized and start to reassociate in a random manner, the cross linked matrix formed by this process entraps CO2 formed during yeast fermentation and is responsible for the raising process. Addition of thioredoxin H to poor quality flours improves their quality for the production of bread.
Thioredoxin H has also been shown to inactivate snake and bee venom toxins and has been shown to reduce the allergenicity of cereal proteins. In the later, the process is presumably the same as described above; by reducing the S—S bonds holding the storage protein clusters together they are more susceptible to denaturation and proteolysis in the gut. Thioredoxin H may also be overexpressed in transformed corn kernels and other cereal crops. The wet milling industry, which is primarily focused on starch extraction, steeps corn in liquors of sodium metabisulphite or SO2. Although this has many secondary effects (e.g., suppression of microbial activity), the primary function is to cause a dissociation of the storage proteins which leads to more efficient starch extraction. Small increases in extractable starch translate into significant increases in the profit margins for the wet millers. By overexpressing thioredoxin H in maize kernels and other cereals it may possible to improve starch recoveries, reduce steep times, and reduce or eliminate the use of sulfur compounds in the steeping process. Overexpression of thioredoxin H in maize kernels and other cereals may have the added advantage of reducing the allergenicity of any transgenic protein engineered into cereal crops with high sulfhydryl content.